Request with multipart/form-data returns 415 error

Question

I need to receive this request using Spring:

POST /test HTTP/1.1
user-agent: Dart/2.8 (dart:io)
content-type: multipart/form-data; boundary=--dio-boundary-3791459749
accept-encoding: gzip
content-length: 151
host: 192.168.0.107:8443

----dio-boundary-3791459749
content-disposition: form-data; name="MyModel"

{"testString":"hello world"}
----dio-boundary-3791459749--

But unfortunately this Spring endpoint:

@PostMapping(value = "/test", consumes = MediaType.APPLICATION_OCTET_STREAM_VALUE)
public void test(@Valid @RequestPart(value = "MyModel") MyModel myModel) {
    String testString = myModel.getTestString();
}

returns 415 error:

Content type 'multipart/form-data;boundary=--dio-boundary-2534440849' not supported

to the client.

And this(same endpoint but with the consumes = MULTIPART_FORM_DATA_VALUE):

@PostMapping(value = "/test", consumes = MediaType.MULTIPART_FORM_DATA_VALUE)
public void test(@Valid @RequestPart(value = "MyModel") MyModel myModel) {
    String testString = myModel.getTestString();
}

again returns 415 but, with this message:

Content type 'application/octet-stream' not supported

I already successfully used this endpoint(even without consumes) with this old request:

POST /test HTTP/1.1
Content-Type: multipart/form-data; boundary=62b81b81-05b1-4287-971b-c32ffa990559
Content-Length: 275
Host: 192.168.0.107:8443
Connection: Keep-Alive
Accept-Encoding: gzip
User-Agent: okhttp/3.8.0

--62b81b81-05b1-4287-971b-c32ffa990559
Content-Disposition: form-data; name="MyModel"
Content-Transfer-Encoding: binary
Content-Type: application/json; charset=UTF-8
Content-Length: 35

{"testString":"hello world"}
--62b81b81-05b1-4287-971b-c32ffa990559--

But unfortunately now I need to use the first described request and I can't add additional fields to it.

So, I need to change the Spring endpoint, but how?

Answer 1

You need to have your controller method consume MediaType.MULTIPART_FORM_DATA_VALUE,

@PostMapping(value = "/test", consumes = MediaType.MULTIPART_FORM_DATA_VALUE)
......

You also need to add a MappingJackson2HttpMessageConverter support application/octet-stream. In this answer,

• I configure it by using WebMvcConfigurer#extendMessageConverters so that I can keep the default configuration of the other converters.(Spring MVC is configured with Spring Boot’s converters).
• I create the converter from the ObjectMapper instance used by Spring.

[For more information]
Spring Boot Reference Documentation - Spring MVC Auto-configuration
How do I obtain the Jackson ObjectMapper in use by Spring 4.1?
Why does Spring Boot change the format of a JSON response even when a custom converter which never handles JSON is configured?

@Configuration
public class MyConfigurer implements WebMvcConfigurer {

    @Autowired
    private ObjectMapper objectMapper;

    @Override
    public void extendMessageConverters(List<HttpMessageConverter<?>> converters) {

        ReadOnlyMultipartFormDataEndpointConverter converter = new ReadOnlyMultipartFormDataEndpointConverter(
                objectMapper);
        List<MediaType> supportedMediaTypes = new ArrayList<>();
        supportedMediaTypes.addAll(converter.getSupportedMediaTypes());
        supportedMediaTypes.add(MediaType.APPLICATION_OCTET_STREAM);
        converter.setSupportedMediaTypes(supportedMediaTypes);

        converters.add(converter);
    }

}

[NOTE]
Also you can modify the behavior of your converter by extending it.
In this answer, I extends MappingJackson2HttpMessageConverter so that

• it reads data only when the mapped controller method consumes just MediaType.MULTIPART_FORM_DATA_VALUE
• it doesn't write any response(another converter do that).

public class ReadOnlyMultipartFormDataEndpointConverter extends MappingJackson2HttpMessageConverter {

    public ReadOnlyMultipartFormDataEndpointConverter(ObjectMapper objectMapper) {
        super(objectMapper);
    }

    @Override
    public boolean canRead(Type type, Class<?> contextClass, MediaType mediaType) {
        // When a rest client(e.g. RestTemplate#getForObject) reads a request, 'RequestAttributes' can be null.
        RequestAttributes requestAttributes = RequestContextHolder.getRequestAttributes();
        if (requestAttributes == null) {
            return false;
        }
        HandlerMethod handlerMethod = (HandlerMethod) requestAttributes
                .getAttribute(HandlerMapping.BEST_MATCHING_HANDLER_ATTRIBUTE, RequestAttributes.SCOPE_REQUEST);
        if (handlerMethod == null) {
            return false;
        }
        RequestMapping requestMapping = handlerMethod.getMethodAnnotation(RequestMapping.class);
        if (requestMapping == null) {
            return false;
        }
        // This converter reads data only when the mapped controller method consumes just 'MediaType.MULTIPART_FORM_DATA_VALUE'.
        if (requestMapping.consumes().length != 1
                || !MediaType.MULTIPART_FORM_DATA_VALUE.equals(requestMapping.consumes()[0])) {
            return false;
        }
        return super.canRead(type, contextClass, mediaType);
    }

//      If you want to decide whether this converter can reads data depending on end point classes (i.e. classes with '@RestController'/'@Controller'),
//      you have to compare 'contextClass' to the type(s) of your end point class(es).
//      Use this 'canRead' method instead.
//      @Override
//      public boolean canRead(Type type, Class<?> contextClass, MediaType mediaType) {
//          return YourEndpointController.class == contextClass && super.canRead(type, contextClass, mediaType);
//      }

    @Override
    protected boolean canWrite(MediaType mediaType) {
        // This converter is only be used for requests.
        return false;
    }
}

The causes of 415 errors

When your controller method consumes MediaType.APPLICATION_OCTET_STREAM_VALUE, it doesn't handle a request with Content-Type: multipart/form-data;. Therefore you get 415.

On the other hand, when your controller method consumes MediaType.MULTIPART_FORM_DATA_VALUE, it can handle a request with Content-Type: multipart/form-data;. However JSON without Content-Type is not handled depending on your configuration.
When you annotate a method argument with @RequestPart annotation,

• RequestPartMethodArgumentResolver parses a request.
• RequestPartMethodArgumentResolver recognizes content-type as application/octet-stream when it is not specified.
• RequestPartMethodArgumentResolver uses a MappingJackson2HttpMessageConverter to parse a reuqest body and get JSON.
• By default configuration MappingJackson2HttpMessageConverter supports application/json and application/*+json only.
• (As far as I read your question) Your MappingJackson2HttpMessageConverters don't seem to support application/octet-stream.(Therefore you get 415.)

Conclusion

Therefore I think you can successfully handle a request by letting MappingJackson2HttpMessageConverter(an implementation of HttpMessageConverter) to support application/octet-stream like above.


[UPDATE 1]

If you don't need to validate MyModel with @Valid annotation and simply want to convert the JSON body to MyModel, @RequestParam can be useful.
If you choose this solution, you do NOT have to configure MappingJackson2HttpMessageConverter to support application/octet-stream.
You can handle not only JSON data but also file data using this solution.

@PostMapping(value = "/test", consumes = MediaType.MULTIPART_FORM_DATA_VALUE)
public void test(@RequestParam(value = "MyModel") Part part) throws IOException {

    // 'part' is an instance of 'javax.servlet.http.Part'.
    // According to javadoc of 'javax.servlet.http.Part',
    // 'The part may represent either an uploaded file or form data'

    try (InputStream is = part.getInputStream()) {
        ObjectMapper objectMapper = new ObjectMapper();
        MyModel myModel = objectMapper.readValue(part.getInputStream(), MyModel.class);

        .....
    }
    .....
}

See Also

Javadoc of RequestPartMethodArgumentResolver
Javadoc of MappingJackson2HttpMessageConverter
Content type blank is not supported (Related question)
Spring Web MVC - Multipart